1. Field of the Invention
The present invention relates to a process for producing thin protective polyimide layers on ceramic or high temperature superconductors, in which either the polyamic acids from which the polyimide will be derived, or starting substances for the acids, are applied to the superconductor surfaces and imidated, or polyimide solutions are applied to the superconductor surface and dried at elevated temperature.
2. Description of the Prior Art
It is know that high temperature superconductors exposed to ambient influences such as air, moisture, thermal and mechanical stress, etc., show signs of degradation (cf. D. Majundar, D. Chatterjee, G. Pav.-Pujalt: Physica C, 158(3) (1989)) which manifest themselves in a lowering of the critical temperature. A possible cause is the elimination of oxygen (for example in the case of YBa.sub.2 Cu.sub.3 O.sub.7-x).
It would therefore be desirable to have a process whereby dielectric polymer layers can be applied to the surface of high temperature superconductors for passivation and electrical insulation in order to protect the superconductor from ambient influences while preserving the superconducting properties. Electrical insulation with the aid of dielectric layers permits the use of superconductor parts (for example in magnets and components of electric motors) and of superconductor layers in electronic devices.
Various passivating processes have been described, for example the application of carbon layers by magnetron sputtering in JP 60 145 375 A2 and JP 01 246 105 A2. S. Morohashi, H. Tamura, A. Yoshida and S. Hasuo in Mater. Sci. Eng. A 109 (1989), 321, also describe protection by means of plasma polymerization. Coating with organic resin layers (fluoropolymers, epoxy resins, siloxanes) is described in EP 305 168 A2 and the use of polyimides in JP 63 318 015 A2. A likewise frequently employed method of preventing damaging oxygen elimination is the introduction of polymeric (for example ethylcellulose) or wet chemical materials (for example oxalates) during the making of the ceramic (cf. R. P. Vasquez, M. C. Foote, B. D. Hunt; Appl. Phys. Lett., 55 (1989) 1801). Inorganic protective layers such as noble metals, transition metals, SiO.sub.2, Si.sub.3 N.sub.4 and AlN are described by E. Weschke, C. Laubschat, M. Domke, M. Bodenbach, G. Kaindle, J. E. Ortega and M. Mirand in Z. Phys. B: Condens. Matter, 74 (2) (1989) 191.
The preparation of organic protective layers using vapor deposition techniques is described by R. N. Lamb and M. Grunze in Surf. Sci., 204 (1988) 183 and by J. R. Salem, F. O. Sequeda, J. Duran, W. J. Lee, R. M. Yang in J. Vac. Sci., Technol., A4 (3) (1986) 369.
The preparation of organic protective layers by the Langmuir-Blodgett or Langmuir-Schafer technique is described by M. C. Petty in W. J. Feast and H. S. Munro (Editors) in Polymer Surfaces and Interfaces, J. Willey & Sons, Chichester 1987, pp. 163, and the self-assembly r technique by R. Maoz and J. Sagiv in Thin Solid Films 132 (1985) 135. The application of suitably modified polymethacrylates, polyamides, polyurethanes and polyesters by the Langmuir-Blodgett technique is also discernible from the paper by F. Embs, D. Funhoff, A. Laschewsky, U. Licht, H. Ohst, W. Prass, H. Ringsdorf, G. Wegner and R. Wehrmann in Adv. Mater., 3 (1991) 25. As revealed by EP 02 47 358 A2, polyamic acids can be deposited on the substrate also by the Langmuir-Blodgett or Langmuir-Schafer technique.
Any imidation of polyamic acid layers on high temperature superconductors under a protective gas atmosphere or under reduced pressure, whether the layers are being applied from solution, from the vapor phase or by the Langmuir-Blodgett technique, causes degradation of the polymer, owing to diffusion of lattice oxygen out of the ceramic superconductor, and ultimately the loss of the superconducting properties.
It is an object of the present invention to delineate a process whereby these disadvantages are avoided.